Lloyd Arthur Eshbach (1910–2003)

By this point in the series, these had gotten a little redundant, but still good. The nice thing about this series is that the hero has to be clever, more than he has to be powerful, which is a nice change.

While in Scotland, a man 'accidentally' opens a gate to another world. There he finds creatures from Earth's past, and devilish forces that want to fool him into letting them invade Earth, mostly by tricking him into doing what they want him to do. I found these books interesting and entertaining, and a bit off the beaten path.

The further adventures of a man from Earth, who is drawn, or tricked into opening portals into another world or plane of existence by the forces of evil. Once there, he has to outtwit them not only to survive, but to keep them away from Earth and to get back. I thought these were all interesting.

Don't recall much about this one, by this time the unique features of the setting had worn off a bit. This is a fantasy strongly derived from Celtic mythology, at least as a starting point.